ITHACA, N.Y. — Today, we forge new ground: this is the first time I’ve ever composed a weekly forecast for Ithaca and Tompkins County that has no significant chance of precipitation in it. No washouts, no “chance ofs”, no lake-effect showers or pop-up storms. Remember folks, this is “storm country”. I’ve been doing these weekly weather forecasts for the Voice for three years. To call this rare is an understatement.
For this, you can thanks an expansive, strong area of high pressure, the same one that ended the growing season this weekend for many of our higher elevations. This high pressure will remain in control for much of the week, potentially bouncing its core between stronger lobes within a larger airmass, but what all that portends is a dry week ahead, and warmer as the high shifts eastward.
Your Weekly Weather
It is a quiet and rather cool September Sunday as high pressure sits directly overhead. The high shifted southeastward from Canada, and its clockwise flow channeled colder air from the Canadian shield into the Northeast this weekend, bringing frost to many areas, and hard freezes to the higher elevations. A hard freeze means the growing season for those hillsides and mountaintops is over, a bit on the early side compared to most years. In Ithaca and the valleys, while temperatures dropped well into the 30s, they have not been cold enough to trigger a freeze – hopefully your more sensitive plants and flowers have a few more weeks before we cross that meteorological bridge.
Temperatures have topped out in the low 60s, about ten degrees below normal, and the dry air and lack of clouds will allow for substantial radiational cooling after sunset. Ithaca, the lakeshore and the immediate suburbs can expect a chilly, starlit night with lows in the mid 30s, but there’s enough moisture in the air to keep it from getting any cooler than that – the dewpoint is about 34-35°F, and the air temperature cannot fall lower than the dewpoint. However, some of the outlying areas may fall into the lower 30s, and that will allow frost to develop, so a Frost Advisory has been issued by NWS Binghamton.
Looking at Monday, temperatures will rebound modestly as the core of the high pressure system shifts slightly eastward, allowing some mild southerly flow on its rear flank. This will allow temperatures to top out in the upper 60s with clear or near-clear skies. Monday night will allow for some good radiational cooling conditions, but starting from a higher peak and having those southerly winds will prevent the temperature from getting as cold, with lows Monday night in the upper 30s to around 40°F with clear skies.
Tuesday, the first day of astronomical fall (starting 9:31 AM) will see the core of the high shift southwestward over the Southern Appalachians, as Hurricane Teddy presses against its eastward flank. The storm will likely make landfall on Nova Scotia, but not pose any threat to the Northeastern U.S. The shift between lobes in the high will result in wind shifts from southerly to northwesterly, but that northwesterly wind is being channeled around a more southerly lobe of high pressure, so it’s not cold air. In fact, temperatures will be a little warmer, with sunny skies and highs in the low 70s. This air also has moisture in it, so lows will not be so low Tuesday night, with a few passing clouds and temperatures bottoming out in the upper 40s.
Wednesday will be another pleasant day, with the core of the high now dominated by the southerly lobe, and shifting eastward off the Carolina coast. This will channel in warmer, more moist air from the Southern U.S., and temperatures will head above normal, topping out in the upper 70s with mostly sunny skies. Wednesday night will be mostly clear, with a few more clouds towards morning, and lows in the low 50s.
Thursday is the only day where that call of no chance of precipitation may be questioned. A shortwave will pass to the north on the edge of the high pressure system, but has no moisture and weak atmospheric dynamics. Only if it times its passage to tap into afternoon heating is there a chance for showers, and right now, that’s not looking likely. The most likely result is mostly sunny skies with highs near 80°F. Thursday night will be partly cloudy with a low in the low 50s.
Friday shows a weak cold front passing to the north, which will allow cooler air to filter into the region. Once again, the lack of moisture and weak instability mean some clouds, but not any substantial chance of precipitation. It will be partly cloudy with highs in the low 70s. Friday night will be partly cloudy with lows in the upper 40s.
The weekend is looking dry and pleasant. Saturday and Sunday both look to top out in the upper 70s, with lows in the low 50s, and partly cloudy skies on both days. The models do suggest a significant, long-duration storm system moving in for next Monday, which will likely bring this dry, comfortable spell to an end.
Let’s put some emphasis on that “long-duration” part, because the models show rain would likely persist from Monday PM through Wednesday or early Thursday as we head into October. That said, as the core of the low lingers east of the Southern Tier, its counterclockwise flow will churn mild air into the region from the south. This results in a long-term outlook for warmer but wetter than normal conditions as we close out the month of September. October is expected to be on the warm side of normal, with near-normal precipitation.